Biochemical steriod hormone receptor assays are of proven value in the prediction of clinical endocrine reponse in advanced breast cancer. However, they are costly and time consuming and can only be performed in especially equipped laboratories. In addition, they do not provide information regarding tissue or tumor cell heterogeneity. A valid histological method would circumvent many of the difficulties inherent in biochemistry. The estrogen receptor immunocytochemical assay (ERICA) appears to be such a method. Employing specific, antiestrophilin monoclonal antibodies, ERICA shows significant correlations with biochemical ER values as well as with clinical endocrine response. Further correlations of this nature are needed to provide a large data base. Preliminary evidence suggests a degree of positive interaction between the steriod binding sites detected by earlier histochemical methods (putative type II sites) and the type I site measured by biochemistry and ERICA. These interactions appear to allow for better discrimination in selecting candidates for hormonal therapy as well as in predictions of the disease-free interval and time of survival. Exploration and confirmation of these findings will be undertaken. Identification of progesterone receptors in intact tissue sections will also proceed. The ultimate aim of this project remains the same: to devise and evaluate histological techniques for the detection of steriod hormone binding sites in breast cancer, capable of performance and interpretation in the average hospital pathology laboratory.